Date: Mon, 15 Jun 2015 22:28:57 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andi Kleen <andi@firstfloor.org>, Oleg Nesterov <oleg@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Waiman Long <Waiman.Long@hp.com>
Subject: Re: why do we need vmalloc_sync_all?
Message-ID: <20150615202856.GA13273@gmail.com>
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* Andy Lutomirski <luto@amacapital.net> wrote:

> On Sun, Jun 14, 2015 at 7:47 PM, Andi Kleen <andi@firstfloor.org> wrote:
> > Oleg Nesterov <oleg@redhat.com> writes:
> >>
> >> But again, the kernel no longer does this? do_page_fault() does 
> >> vmalloc_fault() without notify_die(). If it fails, I do not see how/why a 
> >> modular DIE_OOPS handler could try to resolve this problem and trigger 
> >> another fault.
> >
> > The same problem can happen from NMI handlers or machine check handlers. It's 
> > not necessarily tied to page faults only.
> 
> AIUI, the point of the one and only vmalloc_sync_all call is to prevent 
> infinitely recursive faults when we call a notify_die callback.  The only thing 
> that it could realistically protect is module text or static non-per-cpu module 
> data, since that's the only thing that's reliably already in the init pgd.  I'm 
> with Oleg: I don't see how that can happen, since do_page_fault fixes up vmalloc 
> faults before it calls notify_die.

Yes, but what I meant is that it can happen if due to an unrelated kernel bug and 
unlucky timing we have installed this new handler just when that other unrelated 
kernel bug triggers: say a #GPF crash in kernel code.

In any case it should all be mooted with the removal of lazy PGD instantiation.

Thanks,

	Ingo
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